Color image forming apparatuses such as an electrophotographic color copier, a color printer, and a color multifunction peripheral are commonly known as image forming apparatuses. Further, color image forming apparatuses of an intermediate transfer belt type and a direct transfer belt type are commonly known as electrophotographic color image forming apparatuses.
The color image forming apparatuses of the intermediate transfer belt type and the direct transfer belt type include for example four photosensitive drums each bearing one of toner images in respective colors of yellow (Y), cyan (C), magenta (M), and black (Bk). The four photosensitive drums are arranged in tandem in a rotational direction (moving direction) of an endless belt. Therefore, the color image forming apparatuses of the intermediate transfer belt type and the direct transfer belt type are sometimes called tandem-type image forming apparatuses.
A tandem-type image forming apparatus gives a potential to each of photosensitive drums and causes the photosensitive drums to bear toner images in respective colors by electrostatic forces. In a color image forming apparatus of the intermediate transfer belt type, toner images in respective colors are transferred to an intermediate transfer belt as a transfer target, in order, such that the toner images are superimposed on one another. Through the above, a color toner image is formed on the intermediate transfer belt. The color toner image is then transferred from the intermediate transfer belt to a recording medium such as paper. In a color image forming apparatus of the direct transfer belt type, toner images in respective colors are transferred from respective photosensitive drums to a recording medium (transfer target) conveyed by a belt, in order, such that the toner images are superimposed on one another.
The tandem-type image forming apparatus gives a potential to each transfer roller (transfer member) located opposite to a corresponding one of the photosensitive drums when transferring the toner images in the respective colors from the respective photosensitive drums to the transfer target. The toner images in the respective colors are transferred from the respective photosensitive drums to the transfer target by a potential difference (transfer field) between each photosensitive drum and a corresponding one of the transfer rollers. Further, in the tandem-type image forming apparatus, static electricity is eliminated from the respective photosensitive drums after transfer of the toner images in the respective colors to the transfer target by for example irradiating the photosensitive drums with static elimination light.
By the way, in order to improve environment of an office or the like, a charging method that generates a reduced amount of ozone, such as a positive DC charging roller method, has been often employed in recent years as a method for charging photosensitive drums in an electrophotographic image forming apparatus. Through use of positively chargeable photosensitive members and employment of the positive DC charging roller method in a tandem-type image forming apparatus, an amount of generation of ozone can be reduced while securing fine-pixel transfer performance.
However, a DC charging roller method such as the positive DC charging roller method is inferior to a scorotron method in its ability to charge a photosensitive member. Therefore, a charge given to the surface of the photosensitive member by a transfer field cannot be completely canceled in a subsequent charging step and tends to remain on the surface of the photosensitive member. That is, the surface of the photosensitive member cannot be uniformly charged, and a potential difference derived from a previously transferred toner image (image) tends to be generated. In other words, history of the previously transferred toner image (image) tends to remain on the photosensitive member. Therefore, the DC charging roller method tends to cause a phenomenon so called transfer memory (drum ghost) in which the previously transferred toner image (image) is transferred lightly to the transfer target in a subsequent transfer step. As a method for solving the above problem, a method of irradiating a photosensitive drum before transfer of a toner image, i.e., a photosensitive drum bearing a toner image, with static elimination light is known (see for example Patent Literature 1).
An image forming apparatus described in Patent Literature 1 irradiates respective photosensitive drums located upstream and downstream in a moving direction of a belt (moving direction of a transfer target) with static elimination light using a static eliminating substrate located between adjacent photosensitive drums. Through the above, the downstream photosensitive drum is irradiated with the static elimination light after transfer of a toner image and the upstream photosensitive drum is irradiated with the static elimination light before transfer of a toner image. In the following description, static elimination after transfer of a toner image may be referred to as post-transfer static elimination and static elimination before transfer of a toner image may be referred to as pre-transfer static elimination.
The pre-transfer static elimination reduces a potential difference between an imaged portion (portion bearing a toner image) and a non-imaged portion (portion bearing no toner image) on the surface of the photosensitive drum. However, in a configuration in which the pre-transfer static elimination and the post-transfer static elimination are performed using a single static eliminating substrate, the pre-transfer static elimination cannot be performed on the most upstream photosensitive drum in the moving direction of the belt. As a result, at the time of transfer of toner images in respective colors from the respective photosensitive drums to a transfer target, a surface potential of the most upstream photosensitive drum may be higher than surface potentials of the other photosensitive drums. In the above situation, if a potential is given to each transfer roller from a single power source at the time of transfer of the toner images in the respective colors from the respective photosensitive drums to the transfer target, a value of a current flowing into the most upstream photosensitive drum becomes excessively large and values of currents flowing into the other photosensitive drums decrease. Therefore, the transfer memory may occur and density may be insufficient. That is, image quality may be deteriorated.
Therefore, a high-voltage power source is typically provided for each transfer roller (transfer member) to maintain currents flowing into the respective photosensitive drums constant.